The present invention relates to a system for detecting the presence of fuel in an oil tank of a turbojet engine.
Initiation and growth of fire aboard an aircraft can have catastrophic results. On-board fire extinguishers are mandatory, but the possibility of such fires can be minimized by eliminating the possible sources of fire. The fluids which may catch fire in a gas-turbine engine are foremost the fuel and next, the oil. Fuel and oil leaks may arise from poor seals and also from damage to mechanical parts. In order to minimize the possibility of on-board fires, such leaks must be prevented at all costs.
Most of today's modern turbojet engines are equipped with an oil-fuel heat exchanger mounted in the lubrication circuit of the engine. If a leak arises between the fuel and oil circuits, as a rule it will be from the fuel circuit into the oil circuit, due to the higher pressure of the fuel. The consequences of such leakage are dire. Diluting the oil with fuel increases the danger of internal engine fires, especially when the aircraft has just landed on the ground. In this instance, when the engine has been shut down, the air-oil labrynth seals are at their maximum and the engine temperature is still high. In addition, the fuel-diluted oil degrades the lubrication properties of the oil, thereby resulting in increased wear and clearances between mechanical parts. An additional source is the overflowing of the oil tank. If the oil pressure is atmospheric, the oil tank will overflow outwardly and, if the oil tank is at the pressure of an inner engine enclosure, the enclosure will be flooded.
The aircraft crew must be warned by an alarm if there is damage to the fuel-oil heat exchanger so that the affected engine may be stopped in flight. In flight, it is possible to ventilate and thereby eliminate the fuel from the engine enclosures and to cool the engine. Thus, the danger of fire while in flight is reduced, compared to the danger immediately after the engine has been stopped on the ground.
As a rule, the presence of fuel in the oil circuit increases the liquid volume in the oil tank. Known detection systems for detecting fuel in the oil circuit are based upon measuring the variation in liquid volume within the oil tank, or in the enclosure containing the turbine bearings. Of necessity, these known systems require a long response time, since they must also take into account the ordinary variations in oil volume related to temperature variations and to changing engine operating modes which take place even when there is no fuel mixed with the oil. Quite obviously, these known systems do not provide the required accuracy and response time necessary in order to minimize the danger of on-board engine fires.